Angular 2 attribute is undefined on ngOnInit

guys.

I'm new to web developing with AngularJs 2 and I don't have experience with AngularJs 1 too. So, I'm sorry if I'm asking something stupid. I'm stuck on a problem and I googled it but I couldn't find any solutions for it. :/

The problem is:

I'm trying to use in a function called at the end of ngOnInit an attribute of an object that I initialize on ngOnInit, but this object is still undefined, so I get the following exception: